Packet microstructure in Fe-0.2 pct C martensite
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THE
m a r t e n s i t e that f o r m s in low carbon F e - C alloys and s t e e l s divides the p a r e n t austentte g r a i n s into a r r a y s of a p p a r e n t l y p a r a l l e l laths that a r e frequently r e f e r r e d to as packets. The f i r s t t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e studies 1'2 c h a r a c t e r i z e d this m i c r o s t r u c t u r e of low carbon m a r t e n s i t e as needles a r r a n g e d in s h e e t s with 4 to 5 deg difference in orientation, but other evidence shows that within a packet t h e r e may be s e v e r a l different m a j o r orientations of the m a r t e n s t t e l a t h s . Light m i c r o s c o p y 3 indicates that laths of one orientation a r e d i s t r i b u t e d within a m a t r i x of other laths s e p a r a t e d f r o m each other by low angle boundar i e s , and evidence obtained from t r a n s m i s s i o n electron m i c r o s c o p y 4'5 shows that a packet may consist of bundles of slightly m l s o r t e n t e d laths, each bundle being s e p a r a t e d f r o m its neighbors by high angle boundaries. $petch and his c o w o r k e r s 6'7 have shown that the Kurdjumow-Sachs orientation between austenite and m a r t e n s i t e would account for v a r i o u s orientations, including some that a r e t w i n - r e l a t e d , between adjacent m a r t e n s i t e units f o r m e d f r o m a common austenite grain. Examples of sets of adjacent laths in t h r e e orientation c a t e g o r i e s p r e d i c t e d by the KurdjumowSachs r e l a t i o n s h i p w e r e p r e s e n t e d , 7 but the r e s u l t s were not extended to the d i s t r i b u t i o n s of the v a r i o u s o r i e n t a t i o n s throughout a packet of lath m a r t e n s i t e . The s i z e of the laths is also of i n t e r e s t in c h a r a c t e r i z ing the m i c r o s t r u c t u r e of a packet. A number of i n v e s t i g a t o r s 8-11 have r e p o r t e d a v e r a g e lath widths of about 0.2 m i c r o n s in v a r i o u s low carbon s t e e l s , but a b e t t e r r e p r e s e n t a t i o n of lath size within a packet a p p e a r s to be a d i s t r i b u t i o n ranging from many n a r r o w laths 0.1 to 0.2 m i c r o n s in width, well below the resolution of the light m i c r o s c o p e , to o c c a s i o n a l l a r g e laths, up to 2 m i c r o n s in width, that a r e r e a d i l y v i s i b l e in the light mtcroscope,12't3 The p u r p o s e of the investigation d e s c r i b e d in this p a p e r was to provide a m o r e complete and quantitative d e s c r i p t i o n of the m i c r o s t r u c t u r e that makes up a packet of lath m a r t e n s i t e . Two a p p r o a c h e s were used. In the one, an extensive d e t e r m i n a t i o n of lath width d i s tribution within a packet was made f r o m both r e p l i c a C. A. APPLE is Engineer, Homer Research Laboratories, Bethlehem
Steel Corporation, Bethlehem, Pa. 18016. R. N. CARON is Engineer, Metals Research Laboratories,Olin Corporation, New Haven, Conn. 06504. G. KRAUSSis Professo
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